The present invention relates, in general, to devices for fiber optical communication systems. More particularly, it pertains to filters and directional couplers for such systems.
Fiber optical communication systems employ extremely thin fibers of glass, plastic, or other transparent materials. The fibers are dielectric waveguides that are used to transmit electromagnetic energy at optical wavelengths. Optical fibers are broadly classified into two groups: single-mode fibers and multimode fibers. Dispersion, which is the spreading or widening of light pulses traveling along the guide, is considerably smaller in single-mode fibers than in multimode fibers. Since dispersion limits the number of pulses per second that may be transmitted on a fiber of a given length, single-mode fibers must be utilized in fiber optical communications systems with very high bandwidths, specifically, greater than 200 MHz, and for long spans, specifically, greater than 10 km.
Multiplexing, which is the simultaneous transmission of two or more signals or channels over the same transmission medium, promotes the efficient use of a transmission medium by more completely using the available bandwidth. Frequency-division multiplexing ("FDM"), or wavelength-division multiplexing ("WDM") as this technique is referred to in the optical fiber art, is one technique that may be employed to increase the information-carrying capacity of an optical fiber. A number of devices for introducing several distinct optical signals into a single optical fiber and removing them from the far end have been suggested.
For example, a paper entitled "Wavelength Division Multiplexing (WDM) Couplers," which was published in Fiber Optics--Technology '82, SPIE Vol. 326, pages 76-82, discusses and illustrates several optical couplers that may be employed for wavelength-division multiplexing and demultiplexing on multimode fibers. In this paper, FIG. 2 shows a lensed dichroic filter type coupler; FIG. 3 illustrates a prism type coupler; FIG. 4 depicts a diffraction grating type coupler. In addition, Table 3 lists the advantages and disadvantages of these couplers.
Furthermore, the Dec. 15, 1983, issue of Applied Optics discloses, on pages 3913 and 3916, an optical coupler that includes an optical fiber incorporating a diffraction grating. The grating coupler couples signals into and out of an optical fiber, which may be a single-mode fiber. Signals with different wavelengths enter and leave fibers placed at different angles. FIG. 4 on page 3916 shows two variations of the diffraction grating on the optical fiber: In one variation, the grating is formed on a flat grating substrate, which was obtained by mechanically polishing the cladding, while in the other variation, the grating is formed on a cylindrical grating substrate, which was obtained by chemically etching the cladding.
Because of their lenses, mirrors, gratings, and other mechanical components, the devices described above require precise alignments and tight tolerances, hence the attenuation, and therefore the performance, of such devices are unsatisfactory. To obtain and maintain such precise alignments and tight tolerances is often difficult. Moreover, these requirements impose restrictions on the temperature range of such devices. Additionally, due to the complicated mechanical arrangement that must be employed, only a limited number of signals may be multiplexed and demultiplexed with some of the devices. Consequently, other approaches, such as the use of optical directional couplers, have been suggested.
For instance, a paper entitled "Optical Directional Couplers with Variable Spacing," by Talal Findakly and Chin-Lin Chen, which was published in Applied Optics, Volume 17, Number 5, dated Mar. 1, 1978, shows an optical directional coupler with a linearly increasing channel spacing in FIG. 1. The waveguides in the coupler are identical dielectric strips, which are positioned on a LiNbO.sub.3 substrate. This paper generally discusses optical directional couplers that have a variable separation between the waveguides. In such structures, power is coupled efficiently from one guide to the adjacent one provided the propagation constants are identical so that the optical phases remain in step (matched) over the full interaction path.
A paper entitled "Wavelength Selective Distributed Coupling Between Single Mode Optical Fibers for Multiplexing," by O. Parriaux, F. Bernoux, and G. Chartier, which was published in the Journal of Optical Communications, Volume 2, Number 3, indicates that a filter-type optical directional coupler may be fabricated from two polished single-mode fibers that have different intersecting dispersion characteristics. FIG. 1 of this paper illustrates a coupler having two step-index, single-mode fibers with different core radii and index differences.
A paper entitled "Tunable Optical Waveguide Directional Coupler Filter," by R. C. Alferness and R. V. Schmidt, which was published in Applied Physics Letters, Volume 33, Number 2, dated July 15, 1978, schematically depicts an optical waveguide directional coupler filter in FIG. 1. The coupler filter is described as being compatible with single-mode fiber systems and suitable for wavelength-division multiplexing and demultiplexing. The coupler filter includes a coupled pair of strip waveguides that have distinct indexes of refraction. The indexes of refraction have intersecting dispersion curves, as in the paper by Parriaux et al. The coupler filter also includes electrodes located over the waveguides; the center wavelength of the coupler filter is tuned by applying voltage to the electrodes in order to change the difference in the indexes of refraction of the waveguides.
R. C. Alferness and Peter S. Cross describe, in an article entitled "Fiber Characteristics of Codirectionally Coupled Waveguides with Weighted Coupling," which was published in the IEEE Journal of Quantum Electronics, Volume QE-14, Number 11, dated November 1978, coupled optical waveguides with tapered coupling strength. This paper states that studies of corrugated optical waveguides have demonstrated that their filter response may be improved by smoothly weighting the corrugation depth. The adjustment and shaping of a filter's response is sometimes referred to as apodization. This article mentions several taper functions that may be employed to improve the filter response of coupled optical waveguides. FIGS. 1 and 6 of this paper include schematic diagrams of coupled optical waveguides. Furthermore, this article indicates that a phased-matched interaction resulting in a complete power transfer between the optical waveguides may be obtained at a specified wavelength by either (a) making the waveguide propagation constants, which are functions of the wavelength, equal at the specified wavelength or (b) using periodic spatial modulation of the waveguide propagation constants or the coupling coefficient.
The September 1969 issue of The Bell System Technical Journal includes a paper entitled "Some Theory and Applications of Periodically Coupled Waves," by Stewart E. Miller, which notes that a periodic magnitude variation of the coupling between two parallel-traveling optical waves having different phase constants can yield a complete power interchange between the two waves. This paper illustrates, in FIG. 2, two dielectric waveguides that are spatially coupled. The first dielectric waveguide has an index of refraction n.sub.1, and the second dielectric waveguide has an index of refraction n.sub.2. Dielectric sheets, which are labeled n.sub.3, are spaced periodically between the two dielectric waveguides along the length of the coupling region L. This paper also illustrates, in FIG. 5, periodically coupled dielectric waveguides. Again, the first dielectric waveguide has an index of refraction n.sub.1, and the second dielectric waveguide has an index of refraction n.sub.2. The substrate has an index of refraction n.sub.s, which is less than n.sub.1 and n.sub.2.
Michel Digonnet and H. J. Shaw, in a paper entitled "Wavelength Multiplexing in Single-Mode Fiber Couplers," which was published in Applied Optics, Volume 22, Number 3, dated Feb. 1, 1983, discuss wavelength-division multiplexing in single-mode optical fibers. They state that two approaches have been used to provide wavelength-selective coupling: The first approach relies upon the frequency dependence of optical coupling in couplers made of identical fibers; the second approach uses differential waveguide dispersion in couplers made of dissimilar fibers. They note that R. C. Alferness and Peter S. Cross, in the paper mentioned above, theoretically analyzed the second approach for the case of channel waveguide couplers. Their paper describes the wavelength multiplexing characteristics of devices fabricated based upon the first approach, namely, with identical fibers. In particular, their paper shows a coupler that includes two planar substrates, each of which is a polished quartz block with a narrow slot in one face. Each slot, which is concave, receives an optical fiber. FIG. 4 of this paper illustrates the geometry of the coupler. They indicate that the coupler may be tuned by increasing the center-to-center spacing between the fibers or by laterally offsetting the fibers.
A paper entitled "Filter Response of Nonuniform Almost-Periodic Structures," by H. Kogelnik, which was published in The Bell System Technical Journal, Volume 55, Number 1, on Jan. 1, 1976, generally discusses the filter characteristics of nonuniform periodic waveguides and contradirectional waves. This paper states that the filter response may be altered by a gradual tapering in the corrugation or grating strength or by a gradual variation of the effective grating period.
Conventional optical directional couplers have several deficiencies. First, coupling losses are large when a fiber with cylindrical geometry is coupled to a directional coupler with rectangular geometry. Second, known directional couplers are not particularly frequency selective. Consequently, they may be used to multiplex and demultiplex only a few signals in a fiber with a limited bandwidth since the spectral widths of the coupled and uncoupled signals will be large.
Accordingly, a need exists for a low-loss, narrow-band coupler for fiber optical communication systems that enables signals with small spectral widths to be multiplexed and demultiplexed.